Grass mowing machines such as fairway mowers, greens mowers, trim mowers and other turf care products are commonly equipped with several powered implements such as cutting reels. In the past, these machines typically have been powered by internal combustion engines, and the cutting reels or other implements were driven by hydraulic motors. Increasingly, traction drives, cutting reels and/or other implements are being powered by electric motors. An example of an electric motor drive for a reel cutting unit is shown in U.S. Pat. No. 6,946,762. For example, a hybrid grass mowing machine may include an internal combustion engine to provide traction drive and turn an alternator or generator, which provides electrical power to electric reel motors that drive the cutting reels. Electric reel motors may include brush-less permanent magnet motors, commutated electric motors, and/or other electric motors, and may be powered directly by an engine driven alternator system, or by an energy storage device or fuel cell.
As an electrically driven implement such as a cutting reel is started, current draw by the electric motor is high compared to the steady state or running current needed to rotate the cutting reel. During start-up, a high torque is required to provide mechanical power to move the cutting reel. The torque must be sufficient to overcome inertia and system losses including electromagnetic losses of the electric reel motor and associated circuitry. As a result, to start turning the cutting reel, electrical power (the product of current and voltage) applied to the electric reel motor must be substantially larger than the steady state power that is needed after start-up. If voltage is held constant, start-up current for an electric reel motor is substantially higher than the steady state current for the motor. For example, start-up current required for an electric reel motor may be as high as 120 Amps DC.
With a typical alternator, particularly a Lundell-type alternator having an output rectifier, if more current is demanded than the alternator can supply, the alternator's output voltage will collapse, yet the current will not correspondingly increase. The alternator's electrical power output does not stay at the maximum available, but drops precipitously.
Efforts have been made to reduce the high start-up current needed for an electric reel motor. For example, U.S. Pat. No. 6,531,850 relates to a motor control for an electrical implement such as a reel mower that provides load control and eliminates high inrush currents and voltage and current transients, so that the electric reel motors can be started smoothly by bringing the voltage up gradually. Additionally, U.S. Pat. No. 7,007,446 relates to a circuit providing a gradual increase in current applied to an electric reel motor, to enable a gradual start-up of the motor.
When starting several electrically powered implements at the same time, however, the current draw is high. For example, start-up current for five electric reel motors may be five times 120 Amps DC. An alternator sufficient to handle the high starting current drawn by five electric reel motors may be oversized to run the motors in steady state.
For the above reasons, it is desired to provide a control system for starting electrically powered implements that reduces peak current demands on an alternator so that the available power will remain close to the alternator's maximum rating. It is desired to provide a control system for starting electrically powered implements that will reduce the current draw required to start up several electrically powered implements such as cutting reels on a grass mowing machine. It is desired to size an alternator for steady state current in an electrical system that starts and operates cutting reels or other implements. An improved control system is needed for starting electrically powered implements.